Conventionally, a check valve that prevents a fuel tank from bursting or the like by allowing a fuel vapor to flow to the outside when the pressure within the fuel tank rises to a certain value or higher, and prevents the fuel tank from being crushed or the like by allowing outside air to flow into the fuel tank from the outside of the fuel tank when the pressure within the fuel tank drops to a certain value or lower than the atmospheric pressure is mounted on a fuel tank of an automobile or the like.
Patent Document 1 discloses such conventional check valve including a main-casing, a sub-casing, a positive pressure valve and a negative pressure valve. The positive pressure valve and the negative pressure valve are slidably disposed in a valve chamber. The positive pressure valve includes a through hole and a negative pressure valve seat, while the negative pressure valve includes a leg portion to be inserted into the through hole. A cap to be engaged with the leg portion is disposed in the positive pressure valve. A positive pressure valve spring is disposed between the positive pressure valve and the sub-casing, while a negative pressure valve spring is disposed between the positive pressure valve and the cap. A positive pressure flow path allowing a fuel vapor to flow out is formed between an outer circumference of the positive pressure valve and an inner circumference of the main-casing, and disposed on the side outer than the positive pressure valve spring, while a negative pressure flow path allowing outside air to flow in is formed between an inner circumference of the positive pressure valve, and the leg portion and an outer circumference of the cap, and between the through hole and the leg portion, and disposed on the side outer than the negative pressure valve spring.
When the pressure within the fuel tank rises to a certain value or higher, the positive pressure valve is pressed by the fuel vapor and slides to be separated from a positive pressure valve seat against the urging force of the positive pressure valve spring, and the fuel vapor is discharged to the outside of the tank. On the other hand, when the pressure within the fuel tank drops to a certain value or lower, the negative pressure valve slides to be separated from the negative pressure valve seat against the urging force of the negative pressure valve spring, and the outside air is introduced into the fuel tank.